Spatiotemporal Stability of a Femtosecond Hard –X-Ray Undulator Source Studied by Control of Coherent Optical Phonons P. Beaud, * S. L. Johnson, A. Streun, R. Abela, D. Abramsohn, D. Grolimund, F. Krasniqi, T. Schmidt, V. Schlott, and G. Ingold Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland (Received 1 June 2007; published 26 October 2007) We report on the temporal and spatial stability of the first tunable femtosecond undulator hard–x-ray source for ultrafast diffraction and absorption experiments. The 2:5–1  A output radiation is driven by an initial 50 fs laser pulse employing the laser-electron slicing technique. By using x-ray diffraction to probe laser-induced coherent optical phonons in bulk bismuth, we estimate an x-ray pulse duration of 140  30 fs FWHM with timing drifts below 30 fs rms measured over 5 days. Optical control of coherent lattice motion is demonstrated. DOI: 10.1103/PhysRevLett.99.174801 PACS numbers: 41.60.Ap, 42.65.Re, 78.47.+p Pump-probe diffraction and absorption experiments with x rays have the potential to capture transient structures during chemical and physical reactions on the fundamental time scales of atomic motion. At future free-electron-laser (XFEL) facilities laser pump, x-ray probe experiments will be an important class of time-dependent experiments [1– 3]. Accurate knowledge of pulse lengths, relative time delay, and the relative spatial position between the pump and probe beams are critical for successful pump-probe experiments. Although there is considerable experience on these issues with laser-based sources of pulsed light, accelerator-based sources offer new challenges. Demon- strated performance of such sources is scarce but needed for the design of future XFEL experiments. Employing an electro-optical technique, single-shot measurements of the difference in arrival time with a short term accuracy of 60 fs rms has recently been demonstrated at the Sub-Picosecond Pulse Source [4] allowing postsort- ing of time-stamped laser-x-ray pump-probe data [5]. In this case, multishot data acquisition was done within 20 mi- nutes. Nonetheless, anticipating a XFEL pulse length be- low 10 fs, the timing control and resolution required may be achieved only by combining active synchronization and single-shot time-stamping measurements. There is an analogous situation regarding long-term spatial and tem- poral stability at spontaneous, low intensity femtosecond x-ray sources where pump-probe data must be accumu- lated stroboscopically over millions of shots. To evaluate the limits of active stabilization, we have done such an experiment using a tunable femtosecond undulator x-ray source (Fig. 1) recently installed at the Swiss Light Source (SLS) [6], a storage ring with a highly stable electron beam operated in top-up mode and controlled by a fast orbit feedback system. Femtosecond x rays are generated using slice-energy modulation [7–11]. Because both the optical pump and the x-ray probe pulses are derived from the same Ti:sapphire laser, they are inherently synchronized. Femtosecond pulses are initially created in a conven- tional Kerr-lens mode-locked oscillator phase-locked to the fifth submultiple of the synchrotron rf-master oscillator (500 MHz) with an estimated jitter <1 ps. These pulses are split into two parallel optical paths: one for exciting a sample optically (the ‘‘pump’’ branch) and another for modulating the energy of electrons in the storage ring (the ‘‘probe’’ branch). The pump pulses enter a regenera- tive amplifier (115 fs FWHM, 1 kHz, 2.1 mJ, 800 nm) and are transported to the experiment via a 22 m long vacuum pipe. The probe pulses are amplified in two steps: first by a regenerative amplifier and then by a symmetrically pumped 2-pass amplifier (50 fs FWHM, 1 kHz, 5.2 mJ, 805 nm). A nearly diffraction-limited spatial profile (M 2  1:4) for efficient slicing is ensured by cryogenic cooling of the 2-pass amplifier laser crystal. The intense femtosecond laser pulses are focused in vacuum over an optical distance of 45 m to interact with the electrons at the modulator with a Rayleigh length of 0.6 m (beam waist w 0  490 m). The environmental temperature is stabilized to <1  C. To minimize bunch lengthening, the storage ring instal- lations for short pulse generation are placed in a single 11 m long straight section. The associated breaking of the ring periodicity imposed constraints on the electron beam parameters in order to avoid deterioration of the storage ring dynamic aperture [12]. The slicing spectrometer consists of a modulator (wiggler: B w;eff  1:98 T, g  11 mm,  w  138 mm, N w  17 periods) for energy modulation, chicane dipoles for pulse separation, quadrupole-focusing magnets to reduce the beam height, and a radiator [13] (in-vacuum undulator: B u;eff  0:92 T, g  5 mm,  u  19 mm, N u  96 periods) operated at high harmonics providing a monochromatic Si(111) flux of 8  10 12 photons=s in a 0.014% bandwidth (aper- ture 240  54 rad 2 , average current 400 mA). When the modulator magnetic field B w;eff is tuned to resonance  L   w 1  K 2 eff =2=2 2 (laser wavelength  L  805 nm, deflection parameter K eff  0:934B w;eff T  w cm, Lorentz factor   4720) by changing the wiggler gap, the laser induces an energy modulation in the copropagat- ing electron bunch. Optimal energy transfer is achieved if PRL 99, 174801 (2007) PHYSICAL REVIEW LETTERS week ending 26 OCTOBER 2007 0031-9007= 07=99(17)=174801(4) 174801-1  2007 The American Physical Society